Brain plaques, long considered the chief killer of brain cells and the cause of Alzheimer's disease, may actually play a protective role under a new theory that is changing the way researchers think about the disease.

Instead of sticky plaques, free-floating bits of a toxic protein called amyloid beta may be what's killing off brain cells in Alzheimer's patients, U.S. researchers say.

If the theory is right, then drugs that target plaque, including bapineuzumab – being developed by Pfizer, Johnson & Johnson and Elan – may be aiming at the wrong target, they say.